1. Field of the Invention
This invention relates generally to telecommunications, and more particularly, to wireless communications.
2. Description of the Related Art
Advances in wireless technology have transformed mobile communications, leading to widespread acceptance and use of cellular technology. However, increasing system capacity to meet this increased demand while maintaining a quality of services to users of mobile communications systems with a limited number of radio channels presents a constant challenge. To support a range of voice and data communications, as well as video, in mobile communication systems, a geographic service area may be partitioned into a number of cells. Each cell has a cell site (also called a base station) connected to a wireline network. The cell site establishes a wireless link over radio channels with wireless communication devices, such as mobile devices within the cell. The mobile device users or subscribers of a wireless service, may send and receive information (e.g. text, audio, speech, or video) via a Public Switched Telephone Network (PSTN). As the mobile device users move from one cell to another cell, their communications may be handed-off to a new cell without an interruption in the wireless service.
In Global System for Mobile Communications (GSM) systems, cell size is directly correlated to the output power of a base station. The output power signal of a GSM base station may be measured and controlled using power control loops because the output power signal is a constant envelope signal. That is, the actual output power is independent of the modulation content of the GSM signal.
However, in many Third Generation (3G) mobile communication systems, such as a Universal Mobile Telecommunications System (UMTS), the situation is different. The UMTS uses signals with higher modulation schemes. These signals have non-constant power envelopes. Depending on the modulation content, the actual output power may vary. A measure for this variation is the ratio between the output power peaks and the long time average of the signal power. This peak to average ratio can vary by 10 dB for the UMTS and a Code Division Multiple Access (CDMA) 2000 signals. Additionally, the long time average output power may vary with the amount of traffic in the wireless network. Thus, the average output power when no user traffic is present is much lower than the average output power with substantial user traffic. This effect is called power rise. So the actual and the average output power no longer is an accurate measure for the power setting needed to cover a certain cell size.
To address some of the above described problems while measuring and controlling the output power of a non-constant envelope signal, the gain of the most important stages is kept constant. This is mainly done by taking a fraction of the actual output power and comparing it with a fraction of the actual input power using a gain control loop. The gain control loop merely compares the input and output signals, especially when handling a fast time variant signal like a CDMA or a Wideband-CDMA modulated signal. However, the comparison must be done using the same absolute time intervals for the input and output signals, providing a relatively imprecise control of a cell size while using a substantially large number of measurement components in the gain control loop.
One disadvantage of this technique is that every transmitter stage inside the gain control loop adds delay to the output signal. Therefore, a time synchronization of the input signal must be performed prior to the comparison. Thus, may additional delay element may be needed. The second disadvantage is the fact that a constant gain per se does not indicate a constant output signal behavior. That is, an unwanted signal variation at the input signal side is directly transferred to the output signal side. The third disadvantage is the fact that the gain control loop only keeps one of the four transmission parameters constant. But the overall transmission of a chain of multiple radio frequency (RF) stages is determined by all four S-parameters of each stage. Any imperfection in the uncontrolled S-parameter values typically leads to an uncontrolled signal variation on the output signal. However, these variations are not covered by the gain control loop.
The present invention is directed to overcoming, or at least reducing, the effects of, one or more of the problems set forth above.